1. Field of the Invention
The present invention relates to a dielectric ceramic filter, and more particularly, to a dielectric ceramic filter connected to a metal guide can and a conductive guide line for having excellent frequency characteristics.
2. Description of the Related Art
Rapid developments in information and communication technology have placed great demand on high frequency broadband communication systems. The high frequency broadband communication system requires a high frequency filter which can operate at a high power and have superior frequency stability against temperature changes. One such filter is the dielectric ceramic filter, which uses the resonant characteristics of a dielectric resonator. Accordingly, the dielectric ceramic filter has been widely used for high frequency filtering. The dielectric ceramic filter has superior resonance characteristics at high frequencies comparing to a filter using a general LC circuit. Also, the dielectric ceramic filter has superior frequency stability against temperature change and can tolerate a high operating power.
FIG. 1A is a perspective view of a coaxial type dielectric resonator of the related art, and FIG. 1B shows the equivalent circuit of the coaxial type resonator in FIG. 1A. As shown in FIGS. 1A and 1B, the dielectric resonator 10 is a rectangular block made of a dielectric material, having a through hole 11 formed in the log axis of the block. The four side surfaces, one of the top and bottom surfaces of the rectangular dielectric block, and the inner surface of the through hole 11, are coated with a conductive material having proper conductivity such as silver (Ag) or aluminum (Al) by vacuum evaporation. That is, the dielectric resonant filter 10 is operated as an LC resonator 20 shown in FIG. 1B by opening one end and shorting other end of the rectangular dielectric block. An axial direction length of the rectangular dielectric resonator 10 is λ/4 of its resonant frequency.
FIG. 2 shows a conventional assembling type dielectric ceramic filter 30 using the dielectric resonator 10. As shown in FIG. 2, the dielectric ceramic filter 30 includes a microstrip line substrate 35 and a plurality of dielectric resonators 10 arranged on the microstrip line substrate 35. Each of the dielectric resonators 10 includes a coil 32 and a capacitor 33. That is, the dielectric ceramic filter 30 uses capacitive coupling and inductive coupling. However, the dielectric ceramic filter 30 has low insertion characteristics because it uses a simple TEM mode. Also, the dielectric ceramic filter 30 has a narrow usable frequency band because of characteristic high frequency limitations. For example, at more than 5 GHz, the dielectric resonator 10 must have a short length L, which is very difficult to manufacture with sufficient accuracy.
To overcome this disadvantage, another conventional dielectric ceramic filter 40 has been introduced, as shown in FIG. 3. As shown in FIG. 3, the conventional dielectric ceramic filter 40 is manufactured by forming a plurality of vertical grooves on both sides of a dielectric block 41, forming a conductive layer on the four side surfaces but not the ends of the dielectric block 41, and mounting the dielectric block 41 on a substrate 44 having a microstrip line 44. However, the conventional dielectric resonator filter 40 does not completely overcome the disadvantages of the coaxial type dielectric ceramic filter 30.
Furthermore, the conventional dielectric resonator filter 40 has a problem of an impedance matching between the input and output ends of the dielectric resonator filter 40 and a connection terminal of an external device, which is necessary to obtain sufficient filter characteristics. If the impedance is not accurately matched, excessive signal loss may occur.
The impedance matching problem can be overcome by controlling the length and width of a microwave incident electrode 45 and a microwave incident pattern 46. However, this control is limited in the conventional dielectric ceramic filter 40, since the impedance changes suddenly at the input and output ends where the dielectric material contacts air. Moreover, the filter characteristics such as insertion and attenuation decrease considerably because the electromagnetic field radiates to a space between the electrode and a conductive guide line at the input/output ends when impedance matching is not achieved.